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Abstract
It is great challenge and interesting for researchers to fabricate substrates for enhanced Raman and sensor, and assemble some easy-to-synthesize metallic nanomaterials into controllable nanostructures with special morphologies and arrangements, via alternating current (AC) electric field. The Au-Ag alloy nanoparticles (Au-Ag alloy NPs) colloidal suspension with excellent dispersibility synthesized by wet chemical method, and the morphology of the assembly can be well controlled by regulating the frequency of the AC electric field. Au-Ag alloy nanochains array (Au-Ag ANCs) with dense plasmonic “hot spots” is formed when the AC electric field of 4Vpp-30kHz is applied, which is supported by the result of finite element method (FEM) numerical simulation. Experimental results demonstrate that Au-Ag ANCs show excellent SERS activity: Au-Ag ANCs can detect both Rhodamine 6G (Rh6G) and crystal violet (CV) in the magnitude order of 10−10 M, and the Raman peaks intensity and analyte concentration has a strong linear correlation (R2 is 0.99339 and 0.95916, respectively). Besides, the introduction of Au-Ag ANCs makes the Raman spectra intensity of thiram (a pesticide) with a concentration of 30 ppm on the surface of the blank ITO glass significantly enhanced, and it can detect thiram with a concentration as low as 0.03 ppm. In addition, Au-Ag ANCs substrate exhibits great uniformity and stability, so they have considerable application potential in the field of quantitative detection of trace substances.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface plasmons (SPs) refers to the surface-bound electromagnetic waves formed by the collective oscillation of free electrons in the metal-dielectric interface or nanostructures under the action of an external light field. It can extremely enhance the local optical field on the metal surface and confine the light field energy in the nanometer or even sub-nanometer space [1]. Metallic nanostructures have excellent plasmonic characteristics in the visible and near-infrared bands, which can localize the energy of the light field to the surface of the metal structures, thereby significantly improving the fluorescence and Raman scattering intensity of the surrounding materials [2,3], so it has important research significance and application prospects in the fields of biochemical testing [4–6], food safety [7–9], environmental protection [10,11], optoelectronic devices and plasmonic catalysis [12–16].
Since the discovery of surface enhanced Raman scattering (SERS), it has effectively solved the weak signal, low detection sensitivity, and susceptibility to fluorescence interference in traditional Raman spectroscopy, due to its high sensitivity, high resolution, and low cost. With the development of nanotechnology, the preparation of SERS substrates with high repeatability, high reproducibility, high stability, high sensitivity, large area and low cost, and simple process becomes more and more promising [17–21]. Therefore, preparing metallic nanostructured substrates with controllable morphology through appropriate methods, and applying its special optical response characteristics to achieve effective control of the optical signal of probe molecules, which has great scientific research significance and engineering promotion and application value, has become one of the research hotspots that researchers generally pay attention to [22–26].
An excellent SERS substrate should meet at least the following three requirements: (1) High enhancement factor, (2) Excellent uniformity and reproducibility, and (3) Simple synthesis method. Metallic nanoparticles (NPs) are one of the simplest nanomaterials that support localized surface plasmon resonance (LSPR), and after self-assembly of them, many metallic nanostructures with different morphologies can be obtained, which is an important tool for studying surface plasmons coupling and SERS effect. There are some self-assembly methods that introduce additional unwanted substances during the assembly process. For example, liquid-liquid self-assembly [27] requires the introduction of surfactants, which will inevitably affect the Raman spectrum of target molecules to be tested. However, the external applied electric field orientation method shows great potential to solve this problem. As the name implies, it is a process that uses external fields (such as electric field, magnetic field, fluid field, surface tension field, etc.) to control the directional movement and arrangement of monodisperse NPs in the liquid phase, and forms the assembly with certain rules. Evaporative self-assembly [28] is a common method to arrange NPs by the variety of fluid field in the evaporation process, but it has a long period (possibly several weeks period) and high requirements on ambient temperature and humidity, so it has poor controllability. Nanoparticle assembly by electric field has attracted gradually improved attention in recent years due to its advantages of speed, low cost, simplicity, stability and reliability. K. Watanabe et al. [29] applied alternating electric fields of different intensities and frequencies to AuNPs suspensions and measured the absorption spectra and surface-enhanced Raman effects, revealing that the application of an AC electric field is a powerful tool for control over the plasmonic properties. Hannah Dies et al. [30] assembled AgNPs through an AC electric field and successfully fabricated dendritic nanostructures near the electrode. The results showed that the substrate exhibited excellent SERS performance and was able to detect a variety of target molecules. Generally speaking, Au and Ag are the two most common SERS substrate materials. Furthermore, Au, Ag nanostructures support denser and more magnificent “hot spots”, and these “hot spots” are usually considered to be the main source of the SERS effect. In fact, Ag has poor chemical stability and is more likely to be oxidized by oxygen in the air and lose its activity. Therefore, Au-Ag alloy nanomaterials have both high SERS activity and high chemical stability, and have more extensive applications than single noble metal materials.
In this paper, we take advantage of the dielectrophoretic force of the alloy nanoparticles in an AC electric field to successfully assemble the Au-Ag alloy nanoparticles (Au-Ag alloy NPs) into Au-Ag alloy nanochains array (Au-Ag ANCs), and obtain a substrate with higher SERS activity. We show that the frequency of the AC electric field has a powerful influence on the assembly process of Au-Ag alloy nanoparticles, that is, when the frequency is low, the alloy nanoparticles are attracted to each other under the action of dielectrophoretic force to form nanochains with a number of “hot spots”. With the frequency varies from low to high, the average length of nanochains first increases and then decreases. However, when the frequency of the AC electric field is too high and exceeds a certain value, the alloy nanoparticles will aggregate with each other to form a cluster structure, and the clusters of the alloy nanoparticles will cause the plasmonic “hot spots” to disappear, thereby affecting the SERS activity of the substrate. By adsorbing different concentrations of Rhodamine 6G (Rh6G) and crystal violet (CV) solutions on the substrate and measuring the Raman scattering spectra, we proved that the Au-Ag ANCs substrate has a significant enhancement of the Raman spectra of the analyte. Meanwhile, the spectral intensity and analyte concentration have a strong linear correlation. In addition, the substrate can also detect pesticide residues such as thiram. Because of the simple, fast to fabricate, low environmental requirements, and high detection sensitivity, Au-Ag ANCs substrate has significant potential application value in the detection of trace substances.
2. Method
2.1 Materials
Hydrogen tetrachloroauric acid (HAuCl4·4H2O), silver nitrate (AgNO3), sodium citrate (Na3C6H5O7·2H2O) and thiram are purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (China). The electrode is a pair of silver electrode with a purity of 99.99%. Indium tin oxide (ITO) conductive glass (R=20-25Ω) is purchased from Luoyang Guluo Glass Co., Ltd. (China). Rhodamine 6G (Rh6G) and crystal violet (CV) are purchased from Exciton, USA. Unless otherwise specified, all solutions in the experiment are aqueous solutions, and the water used is deionized (DI) water (resistivity>18M).
2.2 Synthesis of Au-Ag alloy nanoparticles
Au-Ag alloy nanoparticles (Au-Ag alloy NPs) were synthesized according to the following method [31]: DI water was added to an Erlenmeyer flask, heated and stirred in a water bath at 100 °C. The mixed solution of HAuCl4 (0.01 M) and AgNO3 (0.01 M) was added to the conical flask (the ratio of Au and Ag in alloy NPs can be controlled by regulating the ratio of HAuCl4 and AgNO3) and heated for a period of time (make them fully mixed and dispersed). Subsequently, the pre-prepared sodium citrate solution (0.035 M) was poured into an Erlenmeyer flask. The system was stirred and heated, and cooled to room temperature after full reaction. Afterwards, the production was washed with DI water by centrifugation for 3 times. Finally, we collected Au-Ag alloy NPs colloidal suspension and stored it in refrigerator.
2.3 Assembling Au-Ag alloy nanoparticles in AC electric field
As shown in Fig. 1, the preparation of the SERS active substrate and the analyte detection process are as follows. The pre-cut ITO glass substrate with a size of 2.5 cm × 2.5 cm is ultrasonically cleaned in acetone, alcohol and DI water in sequence and placed in an oven to be dried for subsequent use. The cleaned and dried ITO glass is used as the upper and lower electrodes. A weighing paper of a fixed thickness is used as the separation layer between the two electrodes. A certain volume of colloidal Au-Ag alloy NPs is placed between the two ITO glass electrodes. A period of time allows the nanoparticles to settle near the bottom ITO glass. Subsequently, we apply an alternating current (AC) electric field between the two electrodes, and adjust different frequencies and voltages to obtain substrates with different morphologies. and finally dry at room temperature. After being completely dried, the substrate was immersed in different concentrations of analytes (Rh6G, CV and thiram) for 40 min. The analyte molecules will be adsorbed on the surface of the substrate through physical adsorption. After a period of time, the adsorption and desorption of the molecules will reach a dynamic equilibrium. At this time, the substrate can be taken out for the next Raman spectroscopy measurement.
2.4 Characterization
The morphology of the sample is characterized by scanning electron microscopes (S4800 and SU8020, Hitachi, Japan. Sigma 300, ZEISS, Germany). The UV-Vis absorption spectrum is measured by a spectrophotometer (UV-5900PC, Metash, China). During absorption measurement, the sample is dispersed in DI water.
2.5 Raman measurements and spectral processing
Raman spectra were measured by a micro-confocal Raman spectrometer (inVia, Renishaw, UK.). The measurement process is carried out at room temperature and the excitation light is a laser with a wavelength of 532 nm. The objective lens magnification is 50×, and the numerical aperture (NA) is 0.5. The power of the Raman excitation laser was set to 25 mW. The spectra had an accumulation time of 10 s. In order to eliminate the jitter caused by system noise, each Raman spectrum shown in this work is the average of the spectrum measured at several randomly selected points on the same substrate.
3. Results and discussion
First, we synthesized Au-Ag alloy NPs colloidal suspension with remarkable dispersion by wet chemical reduction method. Figure 2(a) shows the UV-Vis absorption spectrum (normalized) of colloidal Au-Ag alloy NPs, and the inset is a photo of the colloidal suspension. The narrow peak of the absorption spectrum indicates that we have successfully synthesized nanoparticles with regular morphology, uniform size and outstanding dispersion. The single peak indicates that gold and silver are uniformly distributed inside the NPs, that is, what we have synthesized is indeed alloy nanoparticles instead of core-shell or other structures. On the assumption that the conversion rate of the reactants is 100%, the concentration of the Au-Ag alloy NPs colloidal suspension we synthesized is 1.24 × 1012 mL-1, which is actually the highest concentration of colloid used in the experiment. If every nanoparticle is idealized as a perfect sphere, we roughly calculate that the average volume occupied by each nanoparticle in the colloidal suspension is about 40,000 times its own volume. In other words, the distance between the nanoparticles in the colloidal suspension is very far in the natural state, so the LSPR coupling effect between different nanoparticles is extremely weak. According to the SEM characterization image, EDS spectrum and mapping image shown in Fig. 2(b), the Au-Ag alloy NPs we synthesized are uniform in size, with an average diameter of 34 nm (The histogram of nanoparticle size distribution, and the distribution of Au and Ag is very uniform, as shown in Fig. 2(d)). 20 μL of Au-Ag alloy NPs colloidal suspension is placed between two pieces of ITO glass, and the optical microscope image of assembly with a certain morphology formed after the action of AC electric field and natural drying is shown in Fig. 2(c).
Fig. 2. Characterization of Au-Ag alloy NPs and assembly. (a) The UV-Vis absorption spectrum (normalized) of colloidal Au-Ag alloy NPs, and the inset shows the photo of the colloidal suspension. (b) SEM characterization image, element mapping image and EDS spectrum of Au-Ag alloy NPs. (c) Optical microscope image of Au-Ag alloy NPs after AC electric field is applied. (d) The nanoparticle size distribution histogram and fitting curve of Au-Ag alloy NPs synthesized by wet chemical method.
Figure 3 shows the SEM images of assembly of Au-Ag alloy NPs under AC electric fields of different frequencies. It can be seen from the characterization results that when the frequency is low (Fig. 3(a)), the mutual attraction between the alloy NPs is low, the nanochains formed are sparse, and the gap between the nanochains and the nanochains is larger, which is not conducive to the formation of “hot spots”. As the frequency increases (Figs. 3(b)-3(d)), the alloy nanoparticles continue to aggregate, and the nanochains gradually approach each other and gradually become longer. However, with the frequency continues to increase (Figs. 3(e)-3(f)), the nanochains will gradually break. When the frequency rises above a certain threshold (Figs. 3(g)-3(h)), Au-Ag alloy NPs will cluster together due to the more powerful interaction force, forming a large-area cluster structure.
Fig. 3. SEM images of the assembly formed by Au-Ag alloy NPs under the control of AC electric field with different frequencies: (a) 30 Hz, (b) 300 Hz, (c) 3kHz, (d) 30kHz, (e) 60kHz, (f) 120kHz, (g) 150kHz, (h) 240kHz. The intensity of the AC electric field applied is 4Vpp (peak-to-peak value).
The Au-Ag alloy nanoparticles synthesized in this experiment will be negatively charged because of the citrate radicals on the surface, which is why the alloy NPs can maintain excellent dispersion and do not form clusters. Due to the thermal motion of liquid molecules, the particles suspended in the liquid will be collided by the liquid molecules from all directions, which leads to the Brownian motion of the particles suspended in the colloid. In addition, the movement of charged metallic nanoparticles in an AC electric field is also affected by dielectrophoresis (DEP), electrophoresis (EP), alternating current electroosmosis (ACEO), and AC electrothermal flow (AC ETF) [30,32]. DEP is defined as the motion of polarized particles in non-uniform electric fields arising from the interaction of electric fields with the net induced charges at the particle/medium interface. Among them, DEP has the most powerful influence on the assembly of Au-Ag alloy NPs. The time-averaged dielectrophoretic force of metallic nanoparticles in a non-uniform electric field is given by [33]:
The relationship between the real part of the C-M factor and the frequency is shown in the Fig. 4. It can be seen from the figure that as the frequency increases, the C-M factor will change sharply in a certain range, which will have a greater impact on the DEP of the nanoparticles. In the experiment, we observed that when the frequency of the electric field exceeds 150 kHz, the nanoparticles will aggregate with each other to form clusters, resulting in deterioration of the Raman activity of the substrate.
Fig. 4. The relationship between the real part of the C-M factor and frequency. The relative permittivity and conductivity of the nanoparticles were set to be 2.5 and 6 × 10−3 S/m, respectively, and the relative permittivity and conductivity of the medium of the solvent were set to be respectively 78.5 and 2 × 10−5 S/m.
Figure 5 shows the Raman spectra of Rh6G with a concentration of 10−7 M on Au-Ag ANCs substrates fabricated by applying AC electric field at different frequencies. The AC electric field intensity applied in the experiment is 4Vpp (peak-to-peak). In order to clearly compare the SERS enhancement of the substrate under different AC electric field frequencies, the intensities of the two peaks at 612 cm-1 and 1360 cm-1 of Rh6G are plotted in Fig. 5(b). It can be seen from the figure that as the frequency of the AC electric field varies, the substrate exhibits various SERS activity. Furthermore, the relative intensities normalized by those obtained at the same wavenumbers under without AC electric field of two Raman peaks (612 cm-1 and 1360 cm-1) are shown in Fig. 5(c). It can be seen that the substrate prepared under the condition of 4Vpp-30kHz has the most excellent SERS activity, and the intensity of the Raman peak is 5 to 6 times that of the case of no electric field.
Fig. 5. The SERS activity of the assembly formed by Au-Ag alloy NPs under the control of AC electric field with different frequencies. (a) Raman spectra of Rh6G with a concentration of 10−7 M on the assembly formed at different frequencies. The intensities (b) of Raman peaks (612 cm-1 and 1360 cm-1) and the relative intensities (c) normalized by those obtained at the same wavenumbers under no application of AC electric field.
Figure 6(a-c) show the SEM characterization images of Au-Ag ANCs fabricated under the conditions of 4Vpp-30kHz in different fields of view. As can be seen from the Fig. 6(a), the nanochains formed under 4Vpp-30kHz have a longer length and the distances between the nanochains are closer. Figures 6(d)-6(f) show the local electromagnetic field distribution of the assembly of Au-Ag alloy NPs with a radius of 34 nm through different arrangements, which are obtained through numerical simulation. The field distribution is simulated by the 3D finite element method (FEM) with the software package COMSOL Multiphysics and the incident light is a plane light wave with a wavelength of 532 nm and polarized in the Y-axis direction propagating along the Z-axis direction. The second row and the third row are the simulated local electric field distribution diagrams of the substrate under different viewing angles. The lower left corner of the picture provides the coordinate axis corresponding to the model. According to the result presented in Fig. 6, compared with the random distribution of nanoparticles, when Au-Ag alloy NPs are arranged into dense and long nanochains, there will be extremely strong local electromagnetic field (“hot spots”) in the nanogaps between the NPs under the excitation of incident light. When the analytes are at these “hot spots”, their Raman scattering spectra will be extremely enhanced [34].
Fig. 6. SEM characterization images of Au-Ag ANCs in different fields of view: (a) large, (b) middle, (c) small. And the local electromagnetic field distribution with different viewing angles of Au-Ag alloy NPs in different arrangements ((d) randomly arranged, (e) sparse and short nanochains, and (f) dense and long nanochains.) under the excitation of plane light waves with a wavelength of 532 nm.
Figure 7(a) is the absorption spectrum of the substrate prepared under the electric field of different frequency measured experimentally. It can be seen from the figure that as the frequency increasing, the plasmon resonance peak of the assembly red shift and broaden. The dotted line in the figure represents the wavelength of the excitation light used in the experiment at 532 nm, which matches the plasmon mode resonance of the substrate prepared under an electric field of 30 kHz, thus leading to the best enhancement with the substrates prepared at 30kHz field. Figure 7(b) is the absorption spectrum of the Au-Ag alloy nanoparticles in different arrangements calculated by COMSOL Multiphysics. It also proves that the red shift and broadening of the plasmon resonance peak occurs with the mutual aggregation of the nanoparticles, which is consistent with the experimental results.
Fig. 7. Experimental and simulated absorption spectra of different substrate. (a) Absorption spectrum of the substrate prepared under the electric field of different frequency measured experimentally. (b) Absorption spectrum of the Au-Ag alloy NPs in different arrangements calculated by COMSOL Multiphysics.
In order to test the detection ability of the Au-Ag ANCs substrate, we measured the Raman spectra of Rh6G at different concentrations (10−7 M to 10−10 M) on the Au-Ag ANCs substrate fabricated under the condition of 4Vpp-30kHz. It can be seen from Fig. 8(a) that when the concentration of Rh6G is reduced to 10−10 M, Au-Ag ANCs still exhibits higher SERS activity, and several Raman peaks of Rh6G at 612, 1179, 1306, 1360, 1505, 1567, 1595, and 1646 cm−1 are still clearly visible although their intensity has been relatively low. The upper and lower curves are respectively the Raman spectra of Rh6G with a concentration of 10−7 M on a substrate with an AC electric field of 4Vpp-30kHz and without electric field applied. The result shows that the application of the AC electric field increases the intensity of the Raman spectrum of the analyte by more than five times. The SERS enhancement factor (EF) is an important indicator to describe the SERS performance of the substrate. An analytical definition of the EF is defined as follows [35]:
where ${I_{SERS}}$ and ${I_{Ref}}$ are the Raman peak intensities of the analyte on SERS substrate (Au-Ag ANCs) and the reference substrate (bare ITO glass), respectively, and ${c_{SERS}}$ and ${c_{Ref}}$ are the concentrations of the analyte on the SERS substrate and the reference substrate, respectively. Here, the EF with a value of 1.44 × 107 is calculated from the Raman spectrum of Rh6G with a concentration of 10−10 M on Au-Ag ANCs, the Raman peak at 612 cm-1 is selected, and the concentration of Rh6G on the reference substrate is set for 10−3 M.Fig. 8. Detection performance of Au-Ag ANCs for Rh6G. (a) Raman spectra of different concentrations of Rh6G on Au-Ag ANCs fabricated at 4Vpp-30kHz. (b) calibration curve based on the intensity of the 1360 cm-1 Raman peak for Rh6G at concentrations from 10−7 M to 10−10 M. (c) Uniformity test of Au-Ag ANCs: Raman spectra of Rh6G with a concentration of 10−7 M at 10 random measurement points in an area of 1mm2 on the substrate, and the intensities of Raman peaks at 612 cm-1 and 1360 cm-1 are displayed.
To examine the quantitative detection capability of the Au-Ag ANCs substrate, we performed a calibration with Rh6G at concentrations from 10−7 M to 10−10 M, as shown in Fig. 8(b). The intensity of a key peak (at 1360 cm−1, assigned to aromatic C-C stretching [36]) is used for quantification. The data shows a strong linear correlation (R2 value of 0.99339) over 4 orders of magnitude. Therefore, it can be concluded that, although SERS is primarily used for identification purposes, the present method has the potential to also be used for the quantitative detection of analytes in a wide range of concentrations.
To clearly demonstrate that the substrate has great reproducibility, we tested the uniformity of the SERS activity over a large area on the Au-Ag ANCs substrate. As shown in Fig. 8(c), within an area of about 1 mm2 on the substrate, the Raman spectra of Rh6G with a concentration of 10−7 M was measured at 10 random points. The intensity of the peaks at 612 cm-1 and 1360 cm-1 were treated statistically for each spectrum. The results show that the relative standard deviation (RSD) of the intensities of the two peaks at 612 cm and 1360 cm are about 23% and 15%, respectively. Although the results are slightly worse than the results obtained by the liquid-liquid two-phase self-assembly, it is still able to meet the requirements in practical applications.
In order to further illustrate that our substrate has excellent SERS performance and the ability to detect multiple molecules, we also measured the Raman spectra of crystal violet (CV) at different concentrations from Au-Ag ANCs, and the corresponding results are shown in Fig. 9. Au-Ag ANCs substrate also shows sensitive detection capability for CV. Experimental results show that it can detect CV molecules with a concentration as low as 10−10 M and the intensity of the Raman peak at 1620 cm-1 also shows a strong linear correlation with the concentration of CV (R2 value of 0.95916). Correspondingly, the concentration of CV on the reference substrate is 5 × 10−3 M, the concentration of CV on Au-Ag ANCs is 10−10 M, and the Raman peak at 1620 cm-1 is selected. We calculated the enhancement factor of CV on the substrate to be 2.99 × 107. The Raman spectra of CV with a concentration of 10−7 M measured at 10 random points in an area of about 1 mm2 on the substrate demonstrates the great reproducibility of the substrate, too. The RSD of the intensities of two characteristic peaks (912 cm-1 and 1620 cm-1) of CV are about 22% and 20%, respectively.
Fig. 9. Detection performance of Au-Ag ANCs for CV. (a) Raman spectra of different concentrations of CV on Au-Ag ANCs fabricated at 4Vpp-30kHz. (b) Calibration curve based on the intensity of the 1620 cm-1 Raman peak for CV at concentrations from 10−7 M to 10−10 M. (c) Uniformity test of Au-Ag ANCs: Raman spectra of CV with a concentration of 10−7 M at 10 random measurement points in an area of 1 mm2 on the substrate, and the intensities of Raman peaks at 912 cm-1 and 1620 cm-1 are displayed.
When other conditions are exactly the same, we compared the SERS activity of the assembly formed by Au-Ag alloy NPs on the ITO glass substrate with or without AC electric field as shown in Fig. 10(a). The upper and lower curves are respectively the Raman spectra of Rh6G with a concentration of 10-7 M on a substrate with an AC electric field of 4Vpp-30kHz and without electric field applied. The result shows that the application of the AC electric field increases the intensity of the Raman spectrum of the analyte by more than five times. Thiram is a widely used pesticide, with potential toxic effects on the liver and reproductive systems at residue concentrations above 7 ppm, according to the US Environmental Protection Agency [37]. We measured the Raman spectra of thiram with a concentration of 30 ppm on a bare ITO glass substrate and thiram with a concentration from 30 ppm to 0.03 ppm on Au-Ag ANCs, as shown in Fig. 10(b-c). Although the 30 ppm thiram on the blank ITO glass has exceeded the relevant safety standards, the Raman signal is too weak to be detected under our measurement conditions. However, even if the concentration of thiram on the Au-Ag ANCs substrate is reduced to 0.03 ppm, the Raman characteristic peak of thiram is still clearly visible.
Fig. 10. The potential application value of Au-Ag ANCs in on-site detection of pesticide residue. (a) Comparison of SERS performance before and after AC electric field: the upper and lower curves are the Raman spectra of Rh6G with a concentration of 10−7 M on a substrate with an AC electric field of 4Vpp-30kHz and without electric field applied, respectively. (b) Comparison of Raman spectra of thiram on a blank ITO glass substrate and Au-Ag ANCs. (c) Raman spectra of different concentrations of thiram on Au-Ag ANCs fabricated at 4Vpp-30kHz. (d) Stability of SERS activity of the Au-Ag ANCs.
Finally, we tested the stability of the substrate. The Raman spectra of Rh6G with a concentration of 10−8 M on freshly fabricated Au-Ag ANCs and Au-Ag ANCs stored (at room temperature and in an open environment) for 17 days are shown in Fig. 10(d). After 17 days, although the SERS activity of Au-Ag ANCs decreased by about 50%, the Raman peak of Rh6G was still clearly distinguishable. Nowadays, miniaturized, intelligent, portable Raman spectrometers have been directly used for on-site analysis. Transportable, mobile, and ultramobile instruments have been produced for measurements outside the laboratory, even in severe conditions [38]. The Au-Ag ANCs substrate prepared by AC electric field is simple, fast, low-cost and easy to recycle and store, which can be used as a substrate for on-site detection of trace residual analyses. At the same time, the ITO glass substrate in our experiment can be easily replaced with other flexible conductive materials (such as ITO conductive film), so that the Au-Ag ANCs substrate can be more conveniently applied to various on-site detection scenarios without pre-processing the sample. In summary, Au-Ag ANCs substrate can basically meet the requirements for on-site detection of trace residual analytes [39].
4. Conclusion
In this study, we demonstrated a simple, rapid and economical method for the fabrication of ultrasensitive SERS active substrate through the assembly of Au-Ag alloy NPs via dielectrophoresis. The frequency dependence of the dielectrophoretic assembly of Au-Ag alloy nanoparticles has been investigated over a wide frequency range. AC electric field control alloy nanoparticles as a fast, effective and tunable assembly method has been proved: substrates with different morphologies are fabricated under different AC electric field frequencies. When an AC electric field with an intensity of 4Vpp is applied and its frequency is 120kHz and below, the Au-Ag alloy nanochain structure is fabricated and the nanochains are long and densely arranged at 30kHz. Due to the distribution of high density plasmonic “hot spots”, Au-Ag ANCs fabricated under the conditions of 4Vpp-30kHz have excellent SERS activity, and the local electromagnetic field distribution simulated by COMSOL Multiphysics is in accordance with the experimental results. The experimental results show that Au-Ag ANCs is able to detect Rh6G and CV in alcohol with a concentration of 10−10 M, and the intensity of the Raman peak has a strong linear correlation with the analyte concentration (R2 is 0.99339 and 0.95916, respectively), which shows that Au-Ag ANCs has potential applications in the quantitative analysis of trace substances. In addition, Au-Ag ANCs can detect thiram (a kind of pesticide molecule) at a concentration of 0.03 ppm while the Raman peak of thiram at a concentration of 30 ppm on the blank ITO glass is too weak to be detected.
Funding
Xi'an University of Posts and Telecommunications Joint Postgraduate Cultivation Workstation (YJGJ201905); Innovation Funds of Graduate Programs of Xi’an University of Posts & Telecommunications (CXJJLY2019064); the Natural Science Basic Research Plan in Shaanxi Province of China (2020GY-101); The Shaanxi province international cooperation and exchange program (2019KW-027); The National Science Foundation of China (12004304, 62005213).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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